1. Field of the Invention
The present invention relates to an ultrasonic atomizing device, more particularly, to an ultrasonic atomizing device which is preferably used for an oil burner and the like.
2. Description of the Prior Art
An ultrasonic atomizing device has been employed, for example, in an oil burner and the like in order to atomize fuel oil and supply it to the combustion chamber of the burner.
The present inventor has proposed in Japanese Patent Application No. 127847/81 an ultrasonic atomizing device utilizing a coupler (horn) as a mechanical vibration connecting member, which is shown in FIG. 1. In the figure, the ultrasonic atomizing device includes an ultrasonic oscillator system 1 which comprises a conic-frustum shaped coupler 2, a piezoelectric oscillator 3 and a circular resonance plate 4. The conic-frustum shaped coupler 2 includes a large cross-sectional surface, a small cross-sectional surface and a conical surface. The piezoelectric oscillator 3 is disc-like and is provided with the respective circular electrode plates (not shown) on its respective surfaces. A pair of feed conductors (also not shown) are attached to respective electrode plates. The piezoelectric oscillator 3 effecting bending vibration and provided with the electrode plates, is attached to the large cross-sectional surface of the coupler 2. The resonance plate 4 whose material is same as that of the coupler 2 is integrally formed with the small cross-sectional surface of the coupler 2. A supporting groove 5 is formed in the coupler 2 and a supporting member 6 is fitted into the supporting groove 5, as shown in FIG. 1. By this supporting member 6, the ultrasonic oscillator system 1 is flexibly supported. The supporting member 6 has an opening 7 through which a blast of wind W is passed. Liquid to be atomized is supplied from one end of a liquid supply pipe 8 to the operative surface of the resonance plate 4, where the atomized liquid particles are mixed with the blast wind W. Since the piezoelectric oscillator 3 is not sealed with a case or the like in the device having the abovementioned structure, it has a disadvantage that dust or the like included in the blast wind is apt to adhere to the surface of the oscillator 3, or liquids to be atomized are apt to invade the surface of the oscillator 3, which impairs the vibration performance of the device or causes a short-circuit of the electrodes.
Therefore, it has been required and considered to make the oscillator 3 be a sealed structure in order to prevent invasion of the blast wind or liquid particles onto the surface of the oscillator 3. However, there has been a problem in the conventional ultrasonic atomizing device such that the total mass of the ultrasonic oscillator system 1 was designed to be supported at the conical surface of the coupler 2. That is, if a sealing member was mounted on a portion other than the supporting portion of the coupler 2 so as to seal up the oscillator 3, its vibration has been apt to be clamped. Moreover, in order to carry out the sealing of the oscillator 3 by mounting the sealing member on the supporting portion of the coupler 2 and to strengthen the mounted portion, it is required that the supporting member be larger in size and more rigid. Also in this case, it occurs the disadvantage that the vibration may be clamped.
On the other hand, when the liquid to be atomized is supplied from the liquid supply pipe 8 to the operative surface of the resonance plate 4, there is a fear that part of liquid may drop from the operative surface before atomization in case where surface tension or viscosity of the liquid is small. As a result, liquid particles of a uniform size cannot be obtained. An usual conventional atomizing device utilizing a coupler has been so designed that liquid particles atomized on the operative surface of the resonance plate be sent out in a desired direction by forced blast. However, at the same time, non-atomized liquid particles dropped from the resonance plate may sometimes be sent out by the forced blast. Especially in the case where the device is applied to an oil burner, it has the disadvantage that liquid particles dropped from the resonance plate may be entered into the combustion chamber, which causes an incomplete combustion.
Moreover, in a conventional ultrasonic atomizing device incorporating a Langevin type ultrasonic oscillator system with a conic-frustum coupler which utilizes a resonance vibration in the longitudinal direction of the coupler, a dimension of the small cross-sectional surface of the coupler has been designed to be smaller compared to a wavelength of ultrasonic waves in order to increase the ratio of a displacement (.xi..sub.2) of a peripheral portion to a displacement (.xi..sub.1) of a central portion of the resonance plate. For example, the ratio .xi..sub.2 /.xi..sub.1 of about 1.64 has been employed in the conventional device. However, the device of this type utilizing the resonance vibration in the longitudinal direction of the coupler requires an accuracy in dimension of longitudinal direction of the coupler, and scattering of the sound velocity due to the coupler material thereof largely affects its vibration performance, which obstructs mass-production and causes an increase in production cost. On the other hand, in an ultrasonic atomizing device employing a coupler to be stimulated in bending vibration, the coupler undergoes a volume variation in the radial direction thereof. As a result, when a dimension of the cross-section of the connecting portion between the coupler and the resonance plate is small, a Q-value and free impedance Z.sub.moo of the device decreases. Accordingly, in this case, vibration energy cannot be sufficiently transmitted from the oscillator to the resonance plate on which atomization is to take place.